Kernel Stack

• https://www.kernel.org/doc/Documentation/x86/kernel-stacks

• Like all other architectures, x86_64 has a kernel stack for every
  active thread.  These thread stacks are THREAD_SIZE (2*PAGE_SIZE) big.
  These stacks contain useful data as long as a thread is alive or a
  zombie.

• While the thread is in user space the kernel stack is empty
  except for the thread_info structure at the bottom.
  

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